The Trestle Trail Bridge, located north of Interstate 80 near Merle Hay Road, appeared to have collapsed near its center sometime Wednesday afternoon.

The bridge is part of the Trestle to Trestle Trail that runs along the former Inter-Urban rail line from Johnston to Des Moines.

Rich Leopold — director of the Polk County Conservation Board, which owns the bridge — said the cause of the collapse is unknown, but ice jams formed around the bridge Wednesday.

The Trestle Trail Bridge in Johnston collapsed Wednesday. The bridge is part of a bike trail located along the former Inter-Urban rail line that passes beneath Interstate Highway 35/80.(Photo: City of Johnston/Special to the Register)

"With these ice flows, you expect some damage, but it just so happens it was the Trestle to Trestle's turn," Leopold said Wednesday night. "We are confident we will have it up by the major summer season. I know this is a very popular trail, so we're gonna get on this as quickly as possible."

The Trestle to Trestle Trail opened in 2007, providing a crossing point beneath Interstate Highway 35/80. It connects with the Inter-Urban and Neal Smith trails near Euclid Avenue, providing access to downtown Des Moines and beyond.

Johnston officials closed the trail leading to the bridge before the collapse. The city also closed the Beaver Creek Natural Resource Area, the Foxboro Trail area near Lew Clarkson Park and the Augustine Trail.

"We've already got people on the scene, trying to assess the damage, to make sure that we don't have more damage as the ice flows continue, but we want to get it up and operational as quickly as possible," Leopold said.

A portion of the Trestle Trail Bridge in Johnston collapsed into Beaver Creek this week, per this photo from the city of Johnston.(Photo: Provided photo/Special to the Register)

One of the bridge's main supports appears to have been "structurally compromised," Leopold said.

"If this happens anywhere else, let us know: Call the sheriff's office or public works or Polk County Conservation so we can get out there and make sure that people's safety is protected as soon as possible," Leopold said.

The National Weather Service's Des Moines office reiterated Johnston's warning to avoid the closed areas, tweeting: "Please heed all trail and road closures and if you encounter a flooded road on your evening commute, find a different route to get home safely."

The Trestle Trail Bridge in Johnston collapsed Wednesday after ice jams from Beaver Creek collected around the bridge supports. One of the bridge's main supports appears to have been "structurally compromised," said Rich Leopold, director of the Polk County Conservation Board.(Photo: City of Johnston/Special to the Register)

A combination of steady rain and melting snow is expected to lead to flooding this week, according to the National Weather Service. Much of Iowa is under either flood watches or warnings, including Polk County, which is under a flood warning until 1 p.m. Thursday.

Another ice jam clogged the Des Moines River near a campsite in Ottumwa, forcing some nearby residents away from their homes.

Flash-flooding following an ice jam is often the main concern for emergency management personnel. The jams are more likely to occur in winding bodies of water, Donavon said.

Wapello County Emergency Management Coordinator Tim Richmond said Tuesday the ice jams that usually move about a mile a day were getting 4 or 5 miles downstream in 24 hours because of this week's rain and mild temperatures.

Ice jams push against the Trestle Trail Bridge, located north of Interstate 80 near Merle Hay Road in Johnston.(Photo: City of Johnston/Special to the Register)

What is an ice jam?

An ice jam happens when a river freezes and thaws. Once the weather warms and a river begins to thaw, water seeps into the ice, breaking apart larger pieces. These shards can pile up, though, creating an icy dam and flooding the surrounding area. Other times, large pieces of ice can get caught in the middle of a river with nowhere to go, having the same effect.

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An ice jam built up on the Des Moines River at Ottumwa, Iowa, on Monday, March 12, 2019, and the damage was still evident a day later. The jam caused the river to flood a campground on the city's west side. Matt Milner/The Ottumwa Courier via AP

An ice jam built up on the Des Moines River at Ottumwa, Iowa, on Monday, March 12, 2019, and the damage was still evident a day later. The jam caused the river to flood a campground on the city's west side. Matt Milner/The Ottumwa Courier via AP

An ice jam built up on the Des Moines River at Ottumwa, Iowa, on Monday, March 12, 2019, and the damage was still evident a day later. The jam caused the river to flood a campground on the city's west side. Matt Milner/The Ottumwa Courier via AP